More severe U.S. storms will come with global warming, NASA researchers say

Friday

Aug 31, 2007 at 12:01 AM

WASHINGTON — As the world warms, the United States will face more severe thunderstorms with deadly lightning, damaging hail and the potential for tornadoes, a trailblazing study by NASA scientists suggests.

SETH BORENSTEIN

WASHINGTON — As the world warms, the United States will face more severe thunderstorms with deadly lightning, damaging hail and the potential for tornadoes, a trailblazing study by NASA scientists suggests.

While other research has warned of broad weather changes on a large scale, like more extreme hurricanes and droughts, the new study predicts even smaller events like thunderstorms will be more dangerous because of global warming.

The basic ingredients for whopper U.S. inland storms are likely to be more plentiful in a warmer, moister world, said lead author Tony Del Genio, a NASA research scientist.

And when that happens, watch out.

"The strongest thunderstorms, the strongest severe storms and tornadoes are likely to happen more often and be stronger," Del Genio said in an interview Thursday from his office at the Goddard Institute for Space Studies in New York. The paper he co-authored was published online this month in the journal Geophysical Research Letters.

Other scientists caution that this area of climate research is too difficult and new for this study to be definitive. But some upcoming studies also point in the same direction.

With a computer model, Del Genio explores an area that most climate scientists have avoided. Simple thunderstorms are too small for their massive models of the world's climate. So Del Genio looked at the forces that combine to make thunderstorms.

A unique combination of geography and weather patterns already makes the United States the world's hottest spot for tornadoes and severe storms in spring and summer. The large land mass that warms on hot days, the contours of the atmosphere's jet stream, the wind coming off the Rocky Mountains and warm moist air coming up from the Gulf of Mexico all combine.

Del Genio's computer model shows global warming will mean more strong updrafts, when the wind moves up and down instead of sideways.

"The consequences of stronger updrafts are more lightning and bigger hail," he said.

On a normal sunny day, updrafts are less than 1 mile per hour. In a big rainstorm that is not severe, it's about 2 mph. In a severe storm they could be 20 to 30 mph. The faster that updraft, the worse the storms.

The Southeast and Midwest lie in the path of most of the most dangerous of these storms.

However, the new study also forecasts danger for the Western United States. It predicts lightning will increase about 6 percent as the amount of carbon dioxide — the chief global warming gas — doubles.

Previous studies have shown that the West will get drier, making it a tinderbox for more wildfires. This study shows that there will be more matches in the form of lightning strikes to start those fires, Del Genio said.

One general benefit of global warming is decreased wind shear, which is the speed of side-to-side wind as the altitude rises, Del Genio said. That would moderate the effects of updrafts.

However, during certain times of the year and under the right conditions in the Midwest and Southeast, wind shear will increase. Combine wind shear and updrafts, and damaging winds result, the scientist said.

Other pending and recent research, especially from the National Oceanic Atmospheric Administration, point in the same general direction, said several scientists who weren't involved in Del Genio's study. But they said research in this area is so new that the NASA study is not the final word.

"It's certainly a plausible result," said Leo Donner, a climate modeling scientist at NOAA's Geophysical Fluid Dynamics Lab in Princeton, N.J. Donner earlier this year came out with a study predicting more heavy rain as temperatures rise.

Harold Brooks, a top scientist at NOAA's severe storms laboratory in Norman, Okla., has soon-to-be-published studies finding results similar to the new NASA study, especially when it comes to hail. Some of the severe hail that should be increasing could be baseball-sized and come down at 100 mph, "falling like a major league fastball," he said.

He said it's not possible to predict more tornadoes will result from climate change, however.

Jerry Mahlman, who used to be NOAA's top climate model expert, said that a decade ago then-Vice President Al Gore asked if global warming could cause more tornadoes. Then as now, Mahlman said that's something that's just too detailed to derive from complex climate models.

Mahlman, a scientist who has long warned about the dire consequences of global warming, cautions against going overboard on climate change links: "I'm beginning to suspect that global warming is dynamically much less sexy than people want it to be."